1. Field of the Art
The present invention relates generally to an electrochemical element, and more particularly to improvements in an electrochemical element having an electrochemical cell of a laminar structure which employs a planar solid electrolyte body.
2. Related Art Statement
There have been known various electrochemical devices, each of which includes an electrochemical element using solid electrolyte. These electrochemical devices are used, for example, as A/F ratio sensors for detecting the oxygen concentration of exhaust gases produced by internal combustion engines of automotive vehicles, or for detecting the concentration of an unburned component in the exhaust gases. A typical example of such sensors incorporates an electrochemical cell which consists of a zirconia ceramic layer functioning as as an oxygen-ion conductive solid electrolyte, and a pair of porous electrodes. With an electric current flowing between the two electrodes, the electrochemical element performs an electrochemical pumping operation well known in the art. Further, one of the two porous electrodes is exposed to a gas to be measured (e.g., exhaust gases) in an external space, through a thin flat space, a porous ceramic layer or other suitable diffusion-resistance means which have a predetermined diffusion resistance to the molecules of a component to be detected, such as oxygen or an unburned component in the exhaust gases. The concentration of oxygen or unburned component in the exhaust gases may be detected by sensing a pumping current flow between the two porous electrodes. Also known in the art are electrochemical elements or detectors similar to the A/F sensor indicated above, which are used to detect water, hydrogen, carbon dioxides, etc., based on an electrochemical pumping and according to the principle of diffusion resistance to the component to be detected, as discussed above.
Such an electrochemical element is shown in U.S. Pat. No. 4,450,065, in which an electrode formed on one surface of a layer of zirconia ceramics or other solid electrolyte is exposed to an atmosphere in a thin flat space formed in the electrochemical element. This thin flat space communicates with an external space in which a gas to be detected (hereinafter referred to as "measurement gas") exists. The thin flat space is formed so as to provide a predetermined diffusion resistance to a component of the measurement gas. Thus, the atmosphere surrounding the above-indicated electrode is controlled by the thin flat space. Namely, the amount of diffusion of the measurement gas into the thin flat space is regulated by the diffusion resistance of the flat space. In this arrangement, an electrochemical pumping operation is effected between the above-indicated electrode or another electrode also exposed to the atmosphere in the thin flat space, and an electrode which is electrochemically connected through a solid electrolyte layer to the electrode exposed to the thin flat space.
In an electrochemical element of the type indicated above, the diffusion resistance of the thin flat space communicating with the external measurement gas space is generally determined by the configuration and dimensions of the flat space, particularly, by the thickness of the space. Conventionally, such a thin flat space formed in the electrochemical element has substantially the same thickness over its entire area, from its outer or peripheral portion to its inner or central portion.